Musannaf Ibn Abi Shayba 3:6300

Waki’ narrated to us, he said: Sufyan narrated to us, from Abdullah ibn Sharik al-Amiri, he said: I heard Zirr ibn Hubaysh saying: When it is the night of the twenty-seventh, then perform ghusl (ritual bathing). And whoever among you is able to delay his breaking of the fast until the suhur (pre-dawn meal), let him do so, and let him break his fast with a mixture of milk and water.